This invention relates to retinoscopes, and more specifically to streak retinoscopes.
Retinoscopy is a technique which employs a retinoscope to provide an objective indication of the refractive error of an eye. A retinoscope projects a patch of light onto the retina of an eye under examination, whilst permitting a user of the device to observe that patch. A streak retinoscope projects an elongated patch of light onto the eye, and includes a mechanism for rotating and focusing the patch. The rotation of the patch is used in the measurement of the axis of astigmatism of an eye under examination.
In a typical retinoscope, light emitted by a source passes through a focusing lens system, and is then reflected by an angled, half silvered or apertured mirror into the eye under observation. The image of the patch of light on the eye is viewed by the user from the opposite side of the mirror. The retinoscope also has a facility for moving the lens system to alter the focus (ie the vergence) of the patch of light on the retina of an eye under observation.
For ease of use, some retinoscopes have a single dual action control member for rotating the patch of light and for altering the position of the lens system. Such a member may, for example, comprise a rotary sleeve directly connected to the lens system and to a holder for a light source for the retinoscope. The sleeve is usually accessible through cut-outs in the outer housing of the retinoscope. Although the sleeve of this design can rotate through a full 360xc2x0, the portions of the body between the cut-outs limit the positions on the retinoscope at which the user can hold the sleeve. Other types of retinoscope have a sleeve which encircles the retinoscope housing, and is directly linked to the lamp holder and lens system through apertures in the housing. Whilst this arrangement provides a user with access to the sleeve through a full 360xc2x0, the cut-outs in the housing limit the extent of available rotational movement.
Other designs of retinoscope avoid this limitation by connecting an external sleeve, which encircles the retinoscope, to a lamp holder through a system of gears, but such an arrangement is relatively complicated and results in the patch of light being rotated in the opposite sense from the sleeve.
According to the invention, there is provided a streak retinoscope for projecting an elongate patch of light onto the retina of an eye under examination, the retinoscope comprising housing means, rotation means contained within the housing means and operable to rotate the patch relative to said eye, adjustable focusing means for focusing said patch on said eye, and a dual action external control member, which encircles at least part of the housing member, for operating the rotation means and adjusting the focusing means, wherein the control member is coupled to the rotation means by a magnetic linkage so as to permit rotation of the control member relative to the housing means through at least one full revolution.
Since the housing means is encircled by the control member, it does not impede access to the latter. Furthermore, the magnetic linkage enables the control member to move the rotation means without making direct mechanical contact with the latter, and thus avoids the need for any connecting gears or connections which limit the amount of allowable rotation of the control member.
Preferably, the control member is rotatably mounted on the housing means, and is also slidable therealong, the arrangement being such that rotation of the member operates the rotation means, whilst sliding movement of the member adjusts the focusing means.
Preferably, the magnetic linkage couples the control member to both the rotation means and the focusing means.
Preferably, the magnetic linkage comprises a first set of magnets connected to the control member (at angularly fixed positions relative thereto) and a second set of magnets connected to the rotation means (at angularly fixed positions) and to the focusing means. Thus, the magnetic linkage also avoids the need to provide a mechanical connection between the control member and the focusing means. Consequently, the focusing means and rotation means can both be situated wholly within the housing means, whilst the control member can be situated entirely outside the housing means.
Preferably, each set of magnets lies on a respective annular path, the two annular paths preferably being substantially concentric.
Preferably, the orientations of the magnets are such that any angular displacement of the first set of magnets relative to the second results in the exertion on the second set of magnetic attractive and repulsive forces tending to move the second set into its original angular position relative to the first set.
To that end, the polar orientations of the magnets in each set relative to the positions of the magnets in the other set preferably alternate.
Preferably, each magnet is so positioned that one of its poles faces the other set, and in this case, for each magnet, the pole facing the other set is of the opposite polarity to the correspondingly positioned pole of each of its immediate neighbours in its set.
Thus, for example, a magnet in the first set which is positioned with its north pole facing radially inwards towards a second set will be situated between two immediately neighbouring magnets, of the first set, whose south poles face radially inwards.
Preferably, there are six magnets in each set.
Preferably, the rotation means-causes the patch of light to rotate by rotating a lamp, and may therefore to advantage comprise a rotatable lamp holder and a rotatable inner sleeve which is angularly fixed to said holder and which carries said second set of magnets.
Conveniently, the control member comprises a cylindrical sleeve, on the inner periphery of which the magnets of said first set are mounted.